LMDS/LMCS typically utilizes cellular topologies to achieve coverage of a given service area. The cell topology is frequently depicted as a circular area with a centrally deposed base station. Typically the cells are 1-5 km in diameter. Customer sites, having terminals to interface with the base station over a wireless link, are located within the circular area. In a large metropolitan area several similar cells are arranged in a fixed pattern to provide wide area coverage.
As will be discussed later a cellular area may be divided into sectors with the base station employing a sectored antenna to provide separate coverage to each sector. The bandwidth, licensed to the service provider operating the system, is divided into radio frequency (RF) carriers to provide better utilization of the available bandwidth. Generally, network interface units (NIUs) at customer sites are in fixed locations within a cell or sector and communicate with the base station in a point to point protocol utilizing highly directional antennas.
Wireless transmission of millimeter wave frequencies travels over a line of sight (LOS) path and any obstacles within the path will influence communication between the base station and network interface units at the customer site. Similarly, any time-dependent attenuation of radio signals within the path will result in service degradation. Time dependent path attenuation is caused principally by rain within a cell or sector although other weather related conditions can have similar influences. Such other conditions include snow, sleet or hail, and wind blown materials such as dust, sand etc. In the following discussion, time dependent path attenuation is referred to as rain fade but this term is intended to include the other sources of attenuation.
In view of the variable nature of rain fade, both in terms of duration and intensity, it is desirable to provide some type of controllable power compensation to the radio signal. In conventional cellular and Personal Communications Services (PCS) systems base station power control is used to overcome Rayleigh fading and inter-cell interference. In those systems the typical power step is 3 dB and makes no consideration of the mobile terminal's dynamic power control mechanism.